x86: fix off-by-one in is_xen_fixed_mfn()

x86: fix off-by-one in is_xen_fixed_mfn()

__2M_rwdata_end marks the first byte after the Xen image, not its last
byte. Subtract 1 to obtain the upper bound to compare against. (Note
that instead switching from <= to < is less desirable, as in principle
__pa() might return rubbish for addresses outside of the Xen image.)

Since the & needs to be dropped from the line in question, also drop it
from the adjacent one.

Reported-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>